A data bus transmits data to and from computing elements and data storage devices using, for error protection, parity bits and check sum symbols. These parity bits and check sum symbols enable the systems to detect all single, double and triple bit transmission error patterns and some patterns with four or more erroneous bits.
Data on the bus is in the form of data bytes (8-bits), and each data byte has associated with it a parity bit. The data bytes and parity bits are transmitted in multiple-byte packets. The packets contain, in addition to the data bytes and parity bits, an associated check sum symbol. The parity bits and the check sum symbol provide what is commonly referred to as "horizontal" and "vertical" parity, respectively. These terms are associated with a conceptual configuration of a packet as a one-byte wide and "n"-byte long block of data, where n is the number of bytes in the packet.
Using prior technology, when an interface between the data bus and the storage devices receives a data packet over the bus for storage on the associated storage device, it checks the individual bytes and the entire packet for errors using the parity bits and the check sum symbol, respectively. If there are no detectable errors, the interface converts the bytes to words and buffers the data words in an associated buffer in random access memory (RAM).
The interface then (i) retrieves from the RAM an appropriate number of data bytes to fill one storage unit on the storage medium, for example, 512 bytes to fill a magnetic disk sector, (ii) encodes the converted data to generate error correction code symbols using an error correction code, and, finally, (iii) sends the data and the error correction symbols as analog signals to the storage device. The interface may also encode the data words and the error correction code symbols to facilitate later demodulation of the stored symbols using, for example, a 2/3 code.
Errors may be introduced into the data bytes between the time they are found to be error free using the parity bits and the check sum symbol and the time they are encoded using the error correction code. For example, as the error-free data are being transferred to the buffer for pre-encoded storage, they may be sent over one or more faulty connections and arrive at the buffer with one or more errors. The interface encodes this erroneous data to generate corresponding error correction symbols and sends them to the storage medium for recording thereon. When the data and error correction code symbols are later retrieved from the storage medium, the interface uses the error correction symbols to correct the data to the pre-encoded state, that is, to the erroneous state in which they arrived at the buffer. It then sends the erroneous data to a requesting device as "error-free" data. What is needed is a mechanism to protect the data from such errors.